1. Technical Field
The present disclosure relates to the reception of signals from satellite communication systems and in particular signals complying with the DVB-S2 standard (Digital Video Broadcasting-Satellite 2nd generation).
2. Description of the Related Art
The DVB-S2 standard makes it possible to transmit one or more audio or MPEG-2 or MPEG-4 video flows, modulated in n-PSK (n-phase Phase-Shift Keying) or n-APSK (n-phase Amplitude and Phase Shift Keying), for example QPSK, 8PSK or 16/32APSK.
A DVB-S2 signal comprises a succession of symbols organized in frames of several thousands of symbols. FIG. 1 shows a DVB-S2 frame. In FIG. 1, the frame TR comprises a 90-symbol header HD and 1440-symbol data blocks DT separated by blocks of 36 pilot symbols PL. The header HD announces the frame organization by specifying the modulation used, i.e., QPSK, 8PSK, 16APSK or 32APSK, a redundancy rate of the data coding among a dozen of possibilities, the frame length between a long frame or a short frame, and the presence or not of pilot blocks PL.
The pilot blocks which are known by the DVB-S2 signal receiver allow the phase of the signal received to be precisely estimated by correlation and thus this phase to be followed even in the presence of significant transmission disturbances. The pilot blocks thus allow a phase noise introduced by frequency changes to which the signal is subjected since its emission to be compensated. The main disturbance to which the signal transmitted is subjected is usually a white and Gaussian noise for a transmission channel of a satellite. The frequency changes to which the signal transmitted is subjected are successively introduced by the transmitter, the satellite transponder, the Low-Noise Block LNB and the receiver tuner. The frequency changes introduce a relatively significant phase noise that the receiver must assess and compensate. In addition, the base frequency of the received signal may vary in a relatively significant range (several MHz) due to the limited precision of some elements of the reception chain, and in particular the block LNB.
The shape of the DVB-S2 signal and the disturbances to be taken into account imply some constraints on the architecture of a DVB-S2 signal demodulator. A carrier frequency offset is compensated for before performing a channel filtering. In fact, if the frequency offset is of 5 MHz and the channel has a 10 MHz width, a channel filtering before compensating the frequency offset would have the effect of replacing 5 MHz of useful frequency band by some noise and/or a band of same width of the adjacent channel. Using the pilot symbols to follow the phase of the received signal supposes knowing the future state of the pilot symbols before being able to decode a symbol. Thus, the signal is delayed by at least one interval between two pilot blocks, i.e., 1440 symbols, before assessing the phase of the data symbols between the two pilot blocks.
FIG. 2 schematically shows an example of a DVB-S2 signal demodulator. In FIG. 2, the demodulator DMD1 receives from a tuner a signal S comprising a component in phase I and a component in quadrature Q. The demodulator comprises an analog-to-digital conversion module ADC, a rough frequency correction module CPC, a channel filtering module CHF, a header and pilot symbol processing module PHP, a carrier frequency offset loop filter CLF, a delay line DLN, a fine phase correction module FPC, an equalization module EQU, a phase detection module PDT and phase loop filter CLP.
The module ADC digitizes the signal S at a high sampling frequency (typically 100 to 150 MHz). The module CPC applies to the digitized signal S a frequency translation to roughly correct the carrier frequency offset. The signal translated in frequency at the output of the module CPC is supplied to the module CHF. The module CHF filters and re-samples the signal translated in frequency to a multiple of the symbol frequency of the signal S. The module CHF thus allows in particular the noise outside a channel to be suppressed. The headers HD of the re-sampled signal may thus be decoded, so as to allow the data transmitted by the signal S to be decoded. The re-sampled signal is thus transmitted to the module PHP. At the same time, the re-sampled signal passes through the delay line DLN delaying the signal for a time so the module PHP can supply information allowing the data of the signal S to be decoded. The module CLF performs filtering the carrier frequency offset measured by the module PHP and supplies a filtered value to the module CPC which uses this value to correct the frequency of the signal. The module PHP uses two successive pilot blocks PL of the signal S to reconstruct the evolution of the phase of the re-sampled signal containing the data to be decoded between the two blocks PL. The phase of this signal may thus be for example obtained by interpolation from the phases of the two successive blocks PL. The delay applied to the signal by the delay line DLN is therefore higher than the number of symbols between two pilot blocks PL. The module PHP supplies a phase correction value which is processed by the module CLP before being supplied to the fine phase correction module FPC. The module FPC corrects the phase of the data symbols coming from the delay line DLN according to the correction values filtered by the module CLP. The module FPC supplies a data signal corrected in phase to the equalization module EQU. The equalization module EQU conventionally comprises an equalizer of the self-adapting type which uses the known or estimated symbols to correct equalization coefficients. In fact, the signal S may have been subjected to harmonic distortions which may have several causes. In particular, these distortions may come from filters of the satellite transponder which introduce a group delay on the edges of the signal spectrum. These distortions may also come from base-band filters between the receiver tuner and the analog-to-digital converter. These distortions may also come from the impedance mismatch in a transmission cable between the receiving satellite dish antenna and the decoder; the mismatch may cause echoes and amplitude and phase disturbances according to the frequency.
The module PHP reconstructs the signal phase between two consecutive pilot blocks by taking phase measurements on these by correlation. In the event of severe harmonic distortions, the phase measurements are also disturbed and the residual phase error is increased, thus decreasing the demodulator performance. In response, the minimum level of the signal-to-noise ratio is often increased to obtain a transmission without error, which reduces the transmission efficiency. In addition, decoding the headers HD may also be disturbed. In some transmission modes without pilots, in particular the modes called ACM (Adaptative Coding and Modulation), the headers may be the only reliable information allowing a possible frequency drift of the block LNB to be followed when the noise level only makes it possible to decode the data with a maximum protection.
To free from severe harmonic distortions to which a DVB-S2 signal may be subjected, it has already been suggested to perform decoding the pilot symbols again after equalization and to process the resulting signal in another phase loop. This solution reveals to be expensive since it includes implementation of another delay line to perform an interpolation between two successive pilot blocks PL after equalizing. Moreover this solution does not improve the headers HD before the decoding thereof. It has also been suggested to perform equalizing before decoding the pilot symbols. At the time of equalization, no known or estimated data are therefore available since they may be affected by a significant phase or frequency error. Updating the equalizer coefficients is performed by so-called “blind” algorithms which are less efficient and slower to converge than self-adapting algorithms using known or estimated data.